Short sharp shocks

In the everyday 'classical' world, energy delivered in a series of short sharp shocks has a more dramatic effect than the same amount of energy delivered continuously. You may wash a car with a water hose, which gives a relatively smooth bombardment of the surface. Sand-blasting, however, is not recommended! What may be the same amount of total energy concentrated in individual grains of sand, gives a series of localised sharp shocks, and may well penetrate and damage the paintwork. There would be even more local damage

if all the energy were concentrated in a single steel pellet, but the rest of the surface would remain unaffected.

How about light shining on a surface? Everyday 'classical' instinct tells us that the distribution of energy will be smooth. This conclusion seems justified in that, for example, it can be tolerated by as delicate a surface as human skin, at least in moderation. One would not expect any effect on a metallic surface except perhaps an increase in temperature if the light were intense enough. It is therefore quite surprising that when surfaces of certain metals such as zinc, rubidium, potassium or sodium are illuminated by light of sufficiently low wavelength, we get what is called the photoelectric effect, an immediate release of electrons which can then be drawn away to produce an electric current.